Theory of surface coils

This treatment of surface coils is divided into the use of surface coils as transmitters only, as receivers only, as transmitters and receivers combined, and on experimental aspects relating to the dependence of the interrogated volume on pulse-sequence parameters [Bosl, Bos2]. 

The spatial distribution of Bi. completely describes the receptivity pattern of the surface coil. This distribution can he obtained for any coil geometry from the Biot-Savart law (2.3.8) l.etl). Field distributions simulated for a one-turn receiver coil are shown in Fig. 9.2.3 [Bos 11. Contours of constant field are shown for the j v plane al c = 0 (Fig. 9.2.3(a)) and for the yz plane at x =0 (Fig. 9.2.3(b)). High values of B, . and thus high signal sensitivity is found near the coil wires. With increasing distance y from the coil the transverse magnetic field falls off rapidly. The shape of the Biyy distribution in the. vy plane is distinctly different from that in they plane. This results from the fact that two transverse components of B exist in the jtv plane but only one in the yz plane. In 

Excitation near resonance converts longitudinal magnetization M into the transverse magnetization Mxy according to 

The flip angle is related to the pulse width tp and on the field distribution BuyiR). Because the latter is proportional to the current I through the coil, the flip angle varies with the distance R of the volume element of interest from the centre of the coil. In return the distribution of flip angles produces a dependence of the excited transverse magnetization on the space coordinates. 

When the same surface coil is used for excitation and for reception, B xy R) enters into the detected signal from the transmitter side as well as from the receiver side. As a result the sensitive volume changes in size compared to excitation or reception only. For a homogeneous sample excited by a single pulse, the distribution of signal amplitude is described by 

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